This invention relates generally to digital amplitude modulators and methods of modulating and, particularly, to binary amplitude modulators useable to modulate an analog amplitude signal in accordance with plural bit words.
Digital amplitude modulators which amplitude modulate analog signals in accordance with digital signals are well known as shown in Timothy p. Hulick, "Digital Amplitude Modulation", Broadcast Engineering, pp. 66-80, December, 1987 and U.S. Pat. No. 4,804,931 issued Feb. 14, 1989 to Hulick, incorporated herein by reference. Reference should be made to these documents for the details of operation of such modulators as described by Hulick. While such digital modulators function sufficiently well to be useful, the theoretical efficiency is not independent of the modulation. They also require signals to be in phase quadrature, such that amplitude and phase errors can lead to significant intermodulation and harmonic distortion.
Referring to FIG. 1, central to the prior art digital modulator of Hulick are "n" quadrature hybrid combined signals where "n" is the number of desired binary bits. An analog to digital (A/D) converter 12 that is driven by the amplitude component of the RF input 13, or audio input 15, determines which of "n" high efficiency amplifiers containing the phase component of the RF, such as pin diode switches (PD SW) 14 are switched "on" to pass the RF from a power splitter 16 to amplifiers A.sub.1 -A.sub.n for summing by quadrature combiners 18. As noted, unlike the 100% efficiency maintained by a Class-D amplifier with collector modulation, the modulator efficiency of the digital modulator of FIG. 1 varies linearly with the modulation index.
Still referring to the prior art digital modulator of FIG. 1, the equations for input power (P.sub.in), output power (P.sub.out) and the efficiency (Eff) can be deduced, as given below: EQU P.sub.in =2 c V.sup.2 M (1) EQU P.sub.out =2 c.sup.2 V.sup.2 M.sup.2 ( 2) EQU Eff=P.sub.out /P.sub.in =c M (3)
where: ##EQU1##
Note that "M" in equations (1-3) can be thought of as the modulation index where 0.ltoreq.M.ltoreq.1, so the efficiency given by equation (3) is a linear function of the modulation index. As a consequence, for the typical 20% average modulation index for voice, the maximum efficiency for c=1 is only 20%.
The efficiency for a two-tone input can be found by substituting M=sin(x) in equations (1-3) where the normalized period of the envelope is pi radians. ##EQU2##
The efficiency is thus identical to the ideal class B amplifier when a large number of bits are used (i.e., c=1 as n.fwdarw..infin.). Although the two-tone waveform efficiency is considerably better than for voice inputs, it should be noted that this 78% efficiency assumes no loss in the hybrid combiners 18. Typical commercial hybrids covering the 30-88 Mhz band have approximately 0.7 dB insertion loss which is equivalent to 85% efficiency. Thus, the resultant theoretical two-tone efficiency decreases to approximately 67% (0.85.times.78%). Moreover, assuming the actual efficiency of the wideband of amplifiers is typically 70%, the combined efficiency degrades approximately to 47% (0.70.times.67%).